An electrochemical double layer is a structure that describes the variation of electric potential near a surface of a solid electrode when it is immersed in an electrolyte. This structure consists of two parallel layers of charge, where the first layer is composed of ions that are absorbed onto the surface of the electrode due to chemical interaction and the second layer is composed of ions that are attracted to the surface via Coulomb force, and it behaves like a capacitor that stores charge energy. Compared to the Li-ion batteries, electrochemical double layer capacitors (EDLC) have much lower energy density. For supercapacitors and EDLCs, no surface redox reactions are involved during charge-discharge cycles. However, they have a very long operational life, up to 500,000 charge-discharge cycles, lower costs, and higher power density due to faster charging—discharging cycles. See Conway, B. E. Electrochemical supercapacitors: scientific fundamentals and technological applications. (1999).
There is a need for an electrode that has larger effective surface area than the electrodes used in conventional EDLCs or conventional supercapacitors in order to increase the energy and power density of the EDLC.